1. Field of the Invention
The present invention relates to a scanning processor for use with Cathode Ray Tubes (CRTs) as used in television and computer monitor displays and, more particularly, to CRTs having display surfaces with a profile that is more flat and square than existing CRTs.
2. Description of the Related Art
FIG. 1 shows the effect of the curvature of the CRT on the distance between the inner phosphor surface of the CRT and the electron gun. If the display surface 100 of the CRT were configured to follow the circumference of an imaginary circle 140 centered at the output of the electron gun 110, then each point on the inner surface of the display surface 100 would be equidistant from the electron gun 110.
However, in general, the display surfaces of CRTs are made to be as flat as possible, within the manufacturing constraints of the vacuum bulb. FIG. 1 shows a typical configuration of a display surface 100 superimposed on the imaginary circle 140.
It can be seen from FIG. 1 that the distance between a point at the center of the display surface is significantly closer—as shown by a first arrow 120—to the electron gun 110, than a point at an outer edge of the display surface. The greater distance is represented by a second arrow 130.
The difference in distance between the different points of the display surface poses problems in beam brightness and focusing. Points closer to the gun require a different focal length than to points further away. Closer points will also tend to be brighter than further away points.
FIG. 2 shows an enlarged view of the display surface of a CRT affected by the described focusing problems. The display surface should be displaying a single horizontal line of uniform thickness. However, due to the differing focal length across the horizontal axis of the display screen, the image of the line tends to taper at the extreme edges of the display surface. The effect is shown in a deliberately exaggerated fashion for clarity, but particularly on larger screens it is noticeable to the human eye.
These problems are exacerbated by recent developments in CRT technology which have yielded so-called minineck tubes having a reduced depth i.e., distance between display surface and electron gun.
To overcome problems associated with the increasing flatness and squareness of CRTs, manufacturers of televisions and monitors attempt to configure their products to dynamically adapt the focusing and brightness for different points on the display surface. Prior art solutions include supplying focus and brightness adjust circuits arranged to accept one or more signals configured to alter focus and brightness characteristics of the displayed image.
However, prior solutions are limited in the amount of correction that they are able to supply. Prior solutions are limited to being able to supply parabolic correction signals, i.e., signals characterized by the mathematical expression x2. The correction signals are applied to bias or modulate the focusing and/or brightness signals. Such correction signals have been acceptable with older types of vacuum tubes, but are not found to adequately remedy the problem with the flatter and squarer tubes now becoming available.
A typical graph showing the dynamic focus voltage required for a particular point at a distance x from the screen center is shown in FIG. 3 for both horizontal 200 and vertical 210 deflections. In the vertical direction, the particular curve 210 shown in FIG. 3 is defined by the relationship:Vdyn—focus—v∝x2 
In the horizontal direction, the relationship is different, as the tube is not symmetrical in the horizontal and vertical directions. The relationship defining horizontal dynamic focus is given by:Vdyn—focus—h∝x2.6 
These figures are exemplary only, and different tubes from different manufacturers can have dynamic focus voltages defined as:Vdyn—focus∝xn where n is generally in the range 2-2.6, although further improvements in tube design may lead to progressively higher values of n.
Prior art systems for dynamic focus correction are limited to correcting for the case where the correction voltage is proportional to the square of the distance from the center point. Since most tubes have a different relationship as defined previously, this approximation results in poor focus and brightness performance, particularly at the extremities of the screen, where the value of x is larger and so exacerbates the problem. Also, since different tubes have different characteristics, each type of tube must be individually optimized, leading to increased design work for each end product.